1. Field of the Invention
The present invention relates generally to angular rate sensors and more particularly to inertial gyroscopes in which the rotor is freely suspended relative to a housing for spinning about a spin axis and is adapted to measure the angular rates of motion of its housing about a pair of orthogonal precession axes, in turn orthogonal to the rotor spin axis. The suspension of the rotor is accomplished using magnetic suspension techniques without physical contact of the rotor with stationary parts of the instrument. In accordance with the invention, the magnetic suspension dimensional parameters are so determined as to achieve substantially zero cross-axis spring rates, whereby very precise input rates may be measured by using torque feed back techniques to maintain the gyro rotor continuously at its null location.
2. Description of the Prior Art
Prior art force or torque feed back angular rate gyroscopic sensors have employed a variety of techniques for achieving a substantially frictionless, neutral spring suspension of the sensitive gyroscope rotor. Typical arrangements are shown in the W. G. Wing U.S. Pat. No. 2,719,291 for a "Rate of Turn Gyroscope", issued Sept. 27, 1955, and in the T. R. Quermann U.S. Pat. No. 3,529,477 for a "Gyroscopic Rotor Suspension", issued Sept. 22, 1970, both assigned to Sperry Corporation. In the Quermann patent, for example, the rotor is suspended on the end of a ball-bearing-journalled, motor-driven shaft by means of a complex system of flexures which tend to provide a substantially zero cross-axis spring rate. Another typical arrangement is represented by the dynamically tuned gimbal suspension, one configuration of which is illustrated in the present assignee's pending application Ser. No. 11,965, filed Feb. 14, 1979 in the name of D. H. Duncan for a "Flexure Assembly for Dynamically Tuned Gyroscopes". All of these flexures are complex, delicate and difficult to fabricate, to assemble, to adjust, and to calibrate and are therefore costly. The calibration is subject to long term drift due to the effects of temperature variations, material stress and strain, vibration, and the like. Other suspension techniques have employed fluid bearing suspensions and expensive electrically and mechanically complex electrostatic suspensions. In most known arrangements, the life and, particularly, reliability are compromised and are limited because of electrical and mechanical complexity and because of mechanically contacting elements and attendant lubrication systems. These life and reliability limitations make the use of such prior art devices in such applications as attitude references for aircraft, satellites, and other long life space vehicles undesirable. With the present invention there is normally no physical contact between rotor and stator portions of the suspension system, obviating the need for lubrication systems; the invention provides a mechanically and electronically simple free rotor suspension system.
Magnetic suspensions for gyroscope rotors have received considerable attention recently, particularly in connection with gyroscopes for use in space vehicles and satellites where their non-contacting and inherently frictionless suspension characteristic contribute to extremely long life expectancy, limited only by the inherently long life expectancy of their associated electronic control systems. Typical magnetic suspensions for gyroscopic rotors are disclosed in the A. V. Sabnis U.S. Pat. No. 3,976,339 for a "Magnetic Suspension Apparatus" issued Aug. 24, 1976 and in the J. R. Dohogne, A. V. Sabnis U.S. Pat. No. 4,090,745 for a "Magnetic Suspension Apparatus with Magnetic Stiffness Augmentation", issued May 23, 1978, both assigned to Sperry Corporation. The teachings of these patents include magnetic suspensions of the radially passive, axially active type wherein the primary rotor load is supported and stably positioned by permanent or passive magnet fields in two substantially spaced radial planes, while the rotor axial position is inherently unstable and is rendered stable by means of a closed-loop servo-controlled electromagnetic field. The substantially widely spaced radially passive, axially active suspensions supporting the gyroscope rotor centrally therebetween, inherently provide the rotor with positive angular stiffness. That is, the two magnetic suspension radial stiffnesses behave like physical springs which oppose any angular motion of the spinning rotor; i.e., a long effective lever arm through which the passive magnetic forces act. This is a desirable characteristic in magnetically suspended reaction wheels, as is discussed by Sabnis et al. While the disclosed systems provide positive stiffness with respect to angular forces, they can be made to exhibit negative stiffness and immediately become angularly unstable.
Other known related configurations demonstrate positive stiffness, such as magnetic suspensions for momentum wheels, reaction wheels, or energy storage wheels for space craft stabilization. A typical magnetically suspended rotary inertial ring-shaped rotor is disclosed in the U.S. Pat. No. 4,000,929 to Studer, which employs a large diameter-to-length ratio of about seven to one to achieve positive angular stiffness and has active and passive suspension elements so located in relation to the ring rotor as to provide active magnetic stabilization forces on the ring rotor along mutually perpendicular diameters of the ring and passive magnetic stabilization with respect to its other axes of freedom. In this axially passive, radially active suspension system, for circumferentially spaced, diametrically opposite suspension stations are provided, the passive suspension forces restraining axial displacement of the ring as well as angular motion of the ring about axes orthogonal to the spin axis. Active magnetic stabilization is provided to maintain the ring rotor fixed radially relative to the stations. Thus, this axially (and angularly) passive, radially active suspension also provides positive stiffness opposing angular motion of the ring about axes perpendicular to the spin axis.
In the foregoing classes of magnetically suspended rotors, the geometric construction parameters and the magnetic flux paths are employed to provide quite substantial rigidity to disturbing torques acting on the spinning rotor. While they succeed in providing properties desirable for use in spinning-mass vehicles for their stabilization and control, they do not provide the desirable zero or substantially zero spring rate characteristics required for precision torque balanced, rate sensing gyroscopes.